The present invention relates to a process for the production of a catalytically-active carbonaceous char, and in particular to a catalytically active char produced at low temperatures and activated at high temperature.
The use of carbonaceous chars as catalysts in chemical reactions is well known. Applications that use catalytic chars to improve reaction rate include, but are not limited to NOx reduction, SOx oxidation, chloramine removal, glyphosate production, peroxide destruction, and metalloid and non-metalloid hydride oxidation. In many of these applications the rate of reaction can be limited by the catalytic activity of the char.
The known techniques for increasing the catalytic activity of carbonaceous chars can generally be categorized into three approaches. One approach involves treating a high-temperature carbonaceous char after the completion of the thermal processing used to produce the char. High temperature thermal processing is usually conducted at temperatures equal to or greater than 700xc2x0 C. to produce a high-temperature char such as an activated carbon or charcoal. Chars produced at temperatures below 700xc2x0 C. are referred to as low-temperature chars. In one example of this prior art, a high temperature char is impregnated with metal ions to improve the catalytic performance of the char in NOx removal applications. It is also known that exposing activated carbon to ammonia in an oxidizing environment increases the catalytic activity of the carbon. Similarly, oxidizing an activated carbon, followed by inert heat treatment to drive off the oxygen-containing groups from the surface of the carbon enhances the catalytic performance in SO2/SO3 conversion. It is known furthermore that catalytic oxidative activity of an activated carbon is improved by first oxidizing the carbon and then heating the oxidized carbon in the presence of nitrogen-containing compounds such as urea or melamine.
A second approach process for increasing the catalytic activity of carbonaceous chars provides a the carbonaceous feedstock which is thermally processed to produce the chars. Those skilled in the art are aware that the catalytic activity is affected by the nitrogen content of the feedstock. For example, pure nitrogen-rich compounds such as hexamethylenetetramine, polyacrylnitrile, and gelatin will, when carbonized and activated, produce carbonaceous chars with catalytic activity. Additionally, admixing ammonium salts with inherently nitrogen-poor feedstock, followed by carbonizing and activating, is known to improve NOx reduction performance of the char beyond what is achieved without the addition of the ammonium salts. Similar improvements in the decomposition of hydrogen peroxide are realized with a char made by admixing a nitrogen-containing compound such as urea with a nitrogen-poor feedstock such as sucrose prior to carbonizing and activating.
A more recent approach for increasing the catalytic activity of carbonaceous chars involves modifications to the thermal processes used to produce the chars. Catalytic activity has been significantly enhanced by carbonization and oxidation of a nitrogen-poor bituminous material followed by impregnation of the resultant low-temperature char with a nitrogen-containing compound prior to or during exposure of the char to temperatures of 700xc2x0C. or greater. The resultant high-temperature carbonaceous char may then be activated to the desired degree by any known technique. In this approach, the benefit of extensive oxidation of the carbonized product prior to impregnation with a nitrogen-containing compound is taught.
The relative catalytic activity of carbonaceous chars has been shown to be accurately and conveniently classified by determining the ability of the chars to catalyze the decomposition of hydrogen peroxide in an aqueous solution, as set forth in U.S. Pat. No. 5,470,748 (1995). The decomposition reaction is exothermic and, therefore, causes an increase in the temperature of the solution. Under a defined set of conditions, the elapsed time to achieve 75% of the temperature change resulting from complete decomposition of the hydrogen peroxide, or xe2x80x9ct-xc2xe timexe2x80x9d, depends solely on the ability of a char to catalyze the hydrogen peroxide decomposition reaction. For instance, two chars may exhibit similar physical absorptive capacity measured in terms of the Iodine Number, for example, yet have significantly different t-xc2xe times. Iodine Number is described in test method TM4 of Calgon Carbon Corporation, Pittsburgh, Pa., and is an indication of the available m2/g surface area of the char. Chars having low t-xc2xe values are known to be useful for NOx reduction, SO2 oxidation, chloramine removal, glyphosate production, peroxide destruction, and metalloid and non-metalloid hydride oxidation.
Thus, it is known that the peroxide decomposition ability of char made from a nitrogen-poor carbonaceous feedstock can be improved by combining said feedstock with nitrogen-containing compounds prior to thermal processing. It is also known that carbonizing a nitrogen-poor carbonaceous feedstock under an oxidizing environment, followed by impregnating the oxidized carbonized product with a nitrogen-containing compound, will enhance peroxide decomposition ability of the final activated char. However, it is not taught by, nor can it be inferred from, the prior art that combining a nitrogen-rich compound with a nitrogen-poor carbonaceous feedstock, and then carbonizing the mixture in an oxidizing environment, will enhance the hydrogen peroxide decomposition ability of the final activated char beyond that achieved without said oxidizing environment or, conversely, without said addition of nitrogen-containing compound. The principal disadvantage expected in such a process would be the oxidation and loss of the nitrogen-containing compound prior to high-temperature treatment, and the consequent need for large amounts of these materials during processing to confer the requisite catalytic activity to the final product. If a unique and significant t-xc2xe benefit could derive from such a process using relatively small amounts the nitrogen-containing compound, the process would have significant cost and performance advantages over the prior art. For example, those prior art methods which rely on treatment with added metals to produce catalytic activity impose a cost burden due to special handling and disposal procedures associated with the metals. Prior art methods which rely on the use of pure nitrogen-rich compounds as feedstocks also bear a cost burden due to the high cost of the feedstock and to the hazard created by large amounts of cyanide and other toxic materials which are invariably produced during thermal processing. Furthermore, prior art processes that use high temperature chars, such as activated carbons and charcoals, as feedstocks are inherently more costly because of the additional process steps needed to confer catalytic activity to the final product.
Accordingly, it is the object of the present invention to provide an improved low-cost process for the production of low-temperature carbonaceous chars having improved catalytic activity that are made from nitrogen-poor char feedstocks without the use of added metals or post-treatments of the high temperature char. It is a further object of the present invention to provide these improved cost and performance advantages through relatively minor and low-cost modifications of both the feedstock and the processing conditions typically used to make high temperature chars.
The present invention comprises a method for the production of low-temperature carbonaceous chars having significant catalytic activity. As used herein, the term low temperature carbonaceous char includes the carbonaceous char prepared in accordance with the present invention, and which can be activated at a temperature in excess of 600xc2x0 C. to produce an activated low-temperature catalytic carbonaceous char. The method includes the steps of combining relatively small amounts of a nitrogen-containing compound with a nitrogen-poor carbonaceous feedstock, carbonizing the mixture at temperatures less than 600xc2x0 C. in an oxidizing environment, and then activating the resultant carbonized/oxidized product at temperatures greater than 600xc2x0 C. Surprisingly, large amounts of the nitrogen-containing compound are not required to compensate for losses during the oxidation step. In fact, for a given amount of the nitrogen-containing compound, higher levels of oxidation actually confer higher levels of catalytic activity to the final product. Oxidation of the said mixture can occur during or after carbonization, and is conducted at a level that is typically well beyond the requirements of activated carbon manufacture. The resultant activated carbonaceous char has been found to have appreciable catalytic activity. Furthermore, any of the known processes or methods for production of catalytically active low-temperature carbonaceous chars can be incorporated into the present invention to further enhance the catalytic activity of the resultant char.
In practice, the amount of the nitrogen-containing compound used in the present invention is typically small, preferably less than 15% by weight of the carbon-containing material or, alternatively, an amount such that the desired level of catalytic activity is exhibited by the resultant activated catalytically active high temperature carbonaceous char.
In a preferred embodiment of the invention, the carbon containing material is coal. The nitrogen-containing compound is any organic or inorganic nitrogen containing compound having at least one nitrogen functionality in which the nitrogen exhibits an oxidation number of less than zero. Examples of such nitrogen containing compounds include urea, melamine, ammonium halides, aniline, gelatin, and polyacrylonitrile. Preferably, the components are pulverized together with a suitable binder such as pitch, if necessary or desired, and the resultant pulverized product is formed into granules, disks, spheres, pellets or like physical forms. The resultant formed material is then carbonized by heating in an oxidizing environment at temperatures less than 600xc2x0 C. The resulting low-temperature carbonaceous char is then thermally activated by exposure of the char to one or more of H2O, CO2, or O2 at temperature greater than 600xc2x0 C. The duration of said exposure is selected such that the resultant activated high-temperature carbonaceous char exhibits an Iodine Number greater than 200 mg/g, where Iodine Number is determined in accordance with test method TM4 of Calgon Carbon Corporation, Pittsburgh, Pennsylvania, and is an indication of the available m2/g adsorptive surface area of the char.
In another preferred embodiment of the invention, the carbonaceous material is a coal to which less than 15% by weight of a nitrogen containing compound having at least one nitrogen functionality in which the nitrogen exhibits an oxidation number of less than zero. These materials are pulverized together with a suitable binder such as pitch, if necessary or desired, and the pulverized product is formed into granules disks, spheres, pellets or like physical forms. The resultant formed material is then carbonized by heating in an oxidizing environment at temperature less than 400xc2x0 C. The resulting low-temperature carbonaceous char is then thermally activated by exposure of the char to any combination of H2O, CO2, or O2 at temperature greater than 600xc2x0 C. The duration of said exposure is selected such that the resultant activated high-temperature carbonaceous char exhibits an Iodine Number greater than 400 mg/g, where Iodine Number is determined in accordance with test method TM-4 of Calgon Carbon Corporation, Pittsburgh, Pa., and is an indication of the available m2/g adsorptive surface area of the char.
In another preferred embodiment of the invention, the carbonaceous material is a coal to which less than 5% by weight of a nitrogen containing compound having at least one nitrogen functionality in which the nitrogen exhibits an oxidation number of less than zero. These materials are pulverized together with a suitable binder such as pitch, if necessary or desired, and the pulverized product is formed into disks, spheres, pellets or like physical forms. The resultant formed material is then carbonized by heating in an oxidizing environment at temperature less than 400xc2x0 C. The resulting low-temperature carbonaceous char is then thermally activated by exposure of the char to any combination of H2O, CO2, or O2 at temperature greater than 600xc2x0 C. The duration of said exposure is selected such that the resultant activated low-temperature carbonaceous char exhibits an Iodine Number greater than 600 mg/g, where Iodine Number is determined in accordance with test method TM4 of Calgon Carbon Corporation, Pittsburgh, Pa., and is an indication of the available m2/g adsorptive surface area of the char.